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Annu. Rev. Astron. Astrophys. 1992. 30:
575-611 Copyright © 1992 by Annual Reviews. All rights reserved |
A correlation between the = 10 µm
mid-infrared and 1415 MHz radio luminosities of Seyfert galaxy nuclei
was discovered by van der Kruit
(1971) and soon extended to the nuclei
of normal spiral galaxies (van der
Kruit 1973). At first both the
infrared and radio emission were thought to be synchrotron radiation
from relativistic electrons accelerated by nuclear
monsters (e.g. massive black holes in Seyfert galaxies or other AGNs).
Then Harwit & Pacini (1975)
proposed that the infrared is thermal
reradiation from dusty H II regions, while the 1415 MHz luminosity is
dominated by synchrotron radiation from relativistic electrons
accelerated in SNRs from the same population of massive stars that heat
and ionize the H II regions. Condon
et al. (1982) classified the
luminous central sources in spiral galaxies as either starbursts or
monsters by their radio sizes and morphologies, and they suggested that
the infrared/radio ratio could be used to distinguish starbursts from
monsters since the correlation was much tighter for nuclear starbursts
than for monsters. Rickard & Harvey
(1984) found that the FIR/radio
correlation applies to the disks of normal galaxies as well.
The real significance of the FIR/radio correlation for normal galaxies
that it is so tight and so universal was not appreciated until the large
IRAS survey appeared. The IRAS flux densities at = 60 µm and
= 100 µm can be used to calculate the quantity FIR
which is the total flux between ~ 40
µm and
~ 120 µm. FIR
measures the majority of the flux reradiated by dust in normal galaxies
(Helou et al. 1988). Helou et al. (1985) defined the parameter
where ~ 1.4 GHz unless otherwise specified, as
a logarithmic measure of the FIR/radio flux-density ratio. The
distribution of q was found to be quite narrow (rms scatter
q
0.2 about the median < q > ~ 2.3 at
= 1.4 GHz) among spiral galaxies in Virgo
(Helou et al. 1985), a large, inhomogeneous sample of spiral and irregular
galaxies (Wunderlich et
al. 1987), Sbc galaxies (Hummel
et al. 1988a), E/S0 galaxies with current star formation (Dressel 1988, Wrobel & Heeschen 1988, 1991), extragalactic FIR sources
selected at
= 60 µm (Condon & Broderick 1986, 1991),
and radio flux-limited samples of normal galaxies (Condon & Broderick 1988, Condon et al. 1991b). The FIR/radio
correlation (Condon et al. 1991a)
for galaxies in the revised IRAS BGS (Soifer et al. 1989) is plotted in
Figure 8.
Figure 8 The FIR/radio correlation for
strong sources selected at
= 60
µm and not containing known monsters (e.g. Seyfert nuclei) or
optically thick to free-free absorption at
=
1.49 GHz. The measurement errors are smaller than the intrinsic scatter
for this sample. Abscissa: log FIR luminosity in solar units. Ordinate:
log 1.49 GHz luminosity (W Hz-1).
Since synchrotron emissivity (power radiated per unit volume) is
proportional to B( + 1)/2
= B(1 +
) for a given
density N0E-
of relativistic electrons (Equation 10), it was
proposed (Hummel 1986, Hummel et al. 1988a) that deviations of
individual galaxies from the average FIR/radio ratio
reflect variations in B(1 +
). The FIR/radio ratio is now known
to be nearly constant over a range of ~ 104 in
Um
B2
~ B(1 +
) (Condon et al. 1991c),
where Um is again the magnetic energy density, so this
does not seem to be the case. If the cosmic-ray production rate is
proportional to LFIR, the FIR/radio correlation
actually requires that the total radio energy emitted per
relativistic electron during its lifetime be
nearly independent of Um but it does not constrain the
instantaneous radio power emitted by each electron. Apparently,
galaxies conspire to fix the ratio of synchrotron energy losses to the
radio-quiet losses from inverse-Compton scattering, bremsstrahlung,
ionization, adiabatic expansion, and escape into intergalactic space.
Several explanations for this conspiracy have been proposed. Völk
(1989) argued that the break frequencies (Equation 12) of normal galaxies
are b
5 GHz; that is, galaxies are ``calorimeters'' or
``beam dumps'' in which relativistic electrons lose most of their energy
to synchrotron and inverse-Compton radiation. The ratio of synchrotron
to total energy loss is therefore ~ Um /
(Um + Ur), where
Ur is the radiation energy density, which
must be nearly constant to save the FIR/radio correlation. Stars are the
dominant source for the radiation energy density Ur in
most normal galaxies at low redshifts, although the independent contribution to
Ur from the cosmic microwave background grows as (1 +
z)4 and might quench the radio emission at redshifts z
1. Turbulence
associated with star formation could amplify the magnetic energy density
(Ko & Parker 1989) until it is
comparable with the interstellar
turbulent pressure Pe =
< v2 > / 3,
where the rms turbulent velocity < v2 >1/2
is about 10 km s-1 in our galaxy.
This mechanism will tend to keep the ratio
Um / (Um + Ur)
constant if the star-formation rate varies. However, it may be
difficult to reconcile both the FIR/radio correlation (which requires
that the radio energy density be roughly proportional to
Ur) and a constant
Um / (Um + Ur) with
minimum energy or equipartition
magnetic fields since the relation between the minimum-energy magnetic
energy density Umin and radio energy density
Uradio is not linear: Umin
Uradio4/7
(Equation 13). Also, the FIR/radio correlation holds at frequencies as
low as 0.151 GHz (Cox et
al. 1988), and the observed nonthermal spectral
indices of normal galaxies well below 5 GHz are not as steep as might be
expected: If
0
2 (Bell 1978) then
1 for
b.
Chi & Wolfendale (1990) investigated models in which relativistic electrons
diffuse out of all but the most luminous galaxies to get nonthermal spectral
indices in better agreement with the observations. They assumed
Um ~ Ur so
Um / (Um + Um) ~ 1/2
automatically. Their assumed diffusion coefficient is proportional to
Um so that cosmic rays escape more quickly from
galaxies with strong magnetic fields,
although Lerche & Schlickeiser
(1980) have argued that the diffusion
coefficient D should decrease with B. Finally, the
radio emitting volume was claimed to be proportional to
LFIR0.9 ± 0.4. Thus, the emissivity
is nearly independent of luminosity, in
contrast with the observation that sources with higher
LFIR have
smaller volumes and very high emissivities (Condon et al. 1990). These
assumptions are so strong that they leave little room for departures
from the FIR/radio correlation, but they may not be valid for real galaxies.
The observed FIR/radio correlation is not precisely linear, especially in optically selected samples containing very low-luminosity galaxies (Fitt et al. 1988, Cox et al. 1988, Devereux & Eales 1989). Galaxies with low FIR luminosities have even lower radio luminosities than expected. Such a nonlinearity would occur if either the FIR or radio luminosity is not directly proportional to the star-formation rate. The two-component model for FIR emission (Helou 1986, Lonsdale-Persson & Helou 1987) includes a warm ``active'' component from dusty molecular clouds heated by the massive young stars responsible for radio emission plus a cool ``cirrus'' component of dust heated by the general interstellar radiation field. If the cirrus component of galaxies in a quiescent phase is heated primarily by the older, radio-quiet stellar population, the cirrus contribution to LFIR should be subtracted to make the FIR/radio correlation linear. Fitt et al. (1988) attempted to correct the FIR/radio correlation by using the observed FIR color temperature to estimate and subtract the FIR emission from the cool (they assumed T ~ 20 K) dust component of each galaxy. Devereux & Eales (1989) argued that the intensity of the radiation field heating the cirrus component is proportional to the blue luminosity, and they subtracted a fixed fraction of the blue luminosity from the FIR luminosity of each galaxy to linearize the FIR/radio correlation.
Alternatively, Chi & Wolfendale
(1990) assumed that the FIR luminosity
is proportional to the current rate of massive star formation and hence
cosmic-ray production, but that the radio luminosity might be deficient in
low-luminosity galaxies because the cosmic rays are more likely to escape
by diffusion (or convection). They corrected the observed radio
luminosities for their estimate of these losses to make the FIR/radio
correlation linear. Condon et
al. (1991a) investigated the FIR/radio
correlation for two well-defined samples of normal galaxies optically
selected spiral galaxies brighter than BT = +12 and
the IRAS
Revised Bright Galaxy Sample with S60 µm 5.24 Jy with
sufficiently accurate 1.49 GHz flux densities for individual measurement
errors in q to be smaller than the intrinsic scatter in the q
distribution. They found that none of the corrections described above
can simultaneously linearize the FIR/radio correlation and reduce the
observed scatter for both galaxy samples. An empirical correction to
q was found that depends on the blue/radio ratio;
it can simultaneously linearize the correlation and reduce the scatter. The
galaxies needing the largest corrections generally have the lowest radio
and infrared luminosities but normal blue luminosities, suggesting that
their current star-formation rates may be lower than the average over the
last ~ 109 yr. Thus, the empirical correction is consistent
with Helou's (1986) two-component
model, but only if the radio-loud
population of massive (M
8 M
) stars
heats both the warm dust in H II regions and
contributes significantly to heating the cirrus dust, which need not be very
cool. Xu (1990) has recently
argued that the cirrus is heated by nonionizing
UV from short-lived stars in the 5-20 M
range. Devereux & Young
(1990)
used the F(H
)/FIR ratio to
suggest that the dust contributing to FIR
is heated primarily by stars with M
6 M
, at least in galaxies with LFIR >
109 L
. Both
LFIR and the production rate of
cosmic rays thus appear to be directly proportional to the recent
formation rate of massive stars in most normal galaxies. However, this
is not sufficient to ensure a tight FIR/radio correlation; it is still
necessary to show that the nonthermal radio energy produced per
relativistic electron does not vary significantly from galaxy to
galaxy. Indeed, cosmic-ray escape or other losses may be
a factor in the high FIR/radio ratios of low-luminosity
(LFIR < 109 L
) galaxies, especially dwarf galaxies with
relatively flat radio spectra (Klein
et al. 1984).